Capacitive discharge ignition system having variable capacitance

ABSTRACT

An ignition system for internal combustion engines utilizing a capacitive discharge to supply energy to the spark plugs. Charging and discharging of the capacitor is controlled by a circuit utilizing a silicon controlled rectifier (SCR) which operates in response to engine driven switching means for turning on and in response to positive back-biasing for turning off. The system includes the capability of connecting additional capacitance into the system for the purpose of supplying additional energy to the spark plugs during engine starting and warmup.

United States Patent [191 CAPACITANCE Christopher A. Jacobs, 3570-1/2Eagle Rock Blvd., Los Angeles, Calif. 90065 Nov. 29, 1972 Inventor:

F iled:

.Appl. No; 310,272

Related US. Application Data Continuation-in-part of Scr. No. 866,626,Oct. 15,

1969, Pat. No. 3,716,037.

References Cited UNITED STATES PATENTS 2/1966 lssler 123/148 E Jacobs 11 Dec.'24, 1974 CAPACITIVE DISCHARGE IGNITION 3,267,329 8/1966 Segal123/148 E SYSTEM HAVING VARIABLE 3,331,986 7/1967 Hardin 123/148 E3,658,044 4/1972 Safstrom 123/148 E Primary Examiner-Charles J. MyhreAssistant Examiner-Ronald B. Cox Attorney, Agent, or FirmChristie,Parker & Hale [57] ABSTRACT An ignition system for internal combustionengines utilizing a capacitive discharge to supply energy to the sparkplugs. Charging and discharging of the capacitor is controlled by acircuit utilizing a'silicon controlled rectifier (SCR) which operates inresponse to engine driven switching means for turning on and in responseto positive back-biasing for turning off. The system includes thecapability of connecting additional capacitance into the system for thepurpose of supplying additional energy to the spark plugs during enginestarting and warmup.

4 Claims, 2 Drawing Figures r I 54 9o 35 Y A CAPACITIVE DISCHARGEIGNITION SYSTEM HAVING VARIABLE CAPACITANCE REFERENCE TO RELATEDAPPLICATIONS This application is a continuation-impart of applicationSer. No. 866,626, filed Oct. 15, 1969, now US. Pat. No. 3,716,037.

DESCRIPTION OF THE PRIOR ART The present invention relates to ignitionsystems for internal combustion engines and in particular to an ignitionsystem utilizing a capacitive discharge to supply energy to the sparkplugs of the engine.

Among the various ignition systems which have been tested and/orproduced as equipment for automobiles, particularly those manufacturedin the United States, the most common system is that generally known asthe Ketteringignition system. In the Kettering system, the

collapse of a magnetic field in the primary winding of primary of theignition coil from a conventional power source such as a 6 or '12 voltwet cell battery. In more recent versions of the Kettering system,electronic elements are utilized to perform certain switching functionsas well as in power supplies which, in some in stances, are beingutilized in such ignition systems to improve operatingcharacteristicsSuch recent versions are generally referred to astransistorized ignition systems.

Still another type of ignition system which has gained some popularityis a system referred to as a capacitive discharge system. In such asystem, rather than utilizing the collapse of amagnetic field to.generate a high voltage pulse at the secondary side of an ignition coil,the

system utilizes a capacitor as the primary energy source. Energy issupplied thereto for storage until released as a high energy pulse tothe spark plugs. Typically, such ignition systems include a vibrator orinverter utilizing vacuum tubes or semiconductor devices in the circuitbetween the battery and the energy storage capacitor. Under control ofthe contact breaker points and an electronic switching device such as asilicon controlled rectifier, the capacitor previously charged by theinverter is then discharged into the primary side of an ignition coilcausing a high voltage pulse to be produced on the secondary side of thecoil and a high voltage spark at the plugs.

Heretofore, capacitive discharge ignition systems have had problemsassociated with them due to the method of discharging the capacitor intothe coil. Such problems are products of the actual design of the systemand are not due to any inherent problem in this type of ignition system.There is no theoretical reason why an ignition system utilizing thedischarge of a capacitor cannot perform at a level of reliability whichis comparable to the best performance of the more conventional systemsthereby making available the superior spark quality of capacitivedischarge ignition systems and their. ability to produce firing of thespark plugs despite-changes in plug condition whether due to age,wetting or fouling in a unit of satisfactory reliability.

- SUMMARY OF PRESENT INVENTIO The present invention provides an ignitionsystem for ,intemal combustion engines comprising a source of electricpower coupled to a first capacitor for storing electric power. A secondcapacitor is connected in parallel circuit relationship with said firstcapacitor. Temperature sensitive switching means are connected in seriescircuit relationship with the second capacitor for disconnecting saidsecond capacitor from the ignition system subsequent to starting andwarmup of the engine. An ignition coil is coupled to the first andsecond capacitors and means is coupled to the ignition coil forproducing an electric spark. An electronic switch is coupled to thefirst and second capacitors'for controlling the discharge and charge ofthe first and second capacitors. Engine driven switching means iscoupled to the electronic switch for periodically operating said switchto produce discharge of the capacitors.

In prior art capacitive discharge ignition systems, significant problemshave been encountered in turning off an electronic switch utilized torelease energy from the energy storage capacitor, in protecting theswitch and in obtaining reliable recharging of the capacitor. In the twogeneral I approaches heretofore adopted, the first has utilized theprinciple of interrupting power from the inverter and the second hasoperated on the principle of back-biasing the switch (which is normallya silicon controlled rectifier) by means of currents which are producedin the ignition coil. Each approach is subject to difficulties. In theformer, it is virtually impossible to accurately know when to turn theinverter on and off and be able to do it rapidly enough. This is due tothe changing inductive properties of the coil and changing enginespeeds. If the inverter is turned 'on too soon after the capacitordischarge, the SCR becomes locked on and the ignition system ceases tooperate. If the tumon is delayed too long, there is insufficient time torecharge the capacitor for satisfactory firing of the next spark plug.The latter approach utilizes the inductance of the ignition coil whichis subject to unpredictable change and, therefore, cannot be relied onto turn off the switch. In addition, a significant change in thecondition of the coil, such as a short across a pair of terminals orwetting of the terminals can also produce a current surge which coulddamage or destroy the electronic switch.

The present invention avoids the problems previously characteristic ofcapacitive'discharge ignition systems and provides a system whichutilizes the good qualitites of a silicon controlled rectifier as theelectronic switch controlling the discharge of energy from the capacitorwhile providing additional supporting circuitry to overcome theaforementioned problems. Shut-off of the silicon controlled rectifier isachieved by a back-bias technique, but, instead of relying only upontransient currents within the ignition coil after a discharge of thecapacitor to accomplish this result, the circuit of the presentinvention provides additional means for insuring that the SCR isback-biased a sufficient length of time to produce shut-off regardlessof changes in the characembodies within it the capability of channelingthe residualenergy which is left in the ignition coil after productionof the spark to begin the recharging process of the energy storagecapacitor. Positive shut-off of the SCR is further assured by theprovision of diode circuitry connected to the silicon controlledrectifier in a specific manner. This diode circuitry also acts to reduceor eliminate any residual magnetism in the core of the ignition coil.Thesystem embodies within itself positive protection for the electronicswitch against the possibility of damage due to sudden current orvoltage surges, occurrences which are a significant cause of SCRfailure. Positive protection against the possibility of the SCR firingat the wrong time is provided by means of a control circuit coupledbetween the breaker points of the ignition system and the semiconductorswitch.

DESCRIPTION OF THE DRAWINGS These and other advantages enumerated abovewill be better understood by reference to the following figures wherein:

FIG. 1 is a circuit diagram of an-ignition system utilizing thecapacitivedischarge circuit of the present invention; and

FIG. 2 is a schematic diagram of the capacitive discharge circuit.

DESCRIPTION OF A SPECIFIC EMBODIMENT A function and circuitdiagram of anignition system is shown in FIG. 1. The positive pole of a battery 11 isconnected by an input connection 12 through an ignition switch 13 to acapacitive discharge ignition system according to the present invention.A first output 9 of capacitive discharge system 10 is connected to theprimary winding of an ignition coil 42 and a second output 44 to theignition or contact breaker points 15. The secondary winding of coil 42is connected to a rotor 21 of a conventional distributor which, in turn,has a plurality of contacts 25, each connected to a spark plug 23. Aswill be described in'more detail in conjunction with FIG." 2, theignition system of FIG. 1 utilizes the discharge of energy from astorage capacitor within system 10 through coil-42 and rotor 21 to thespark plugs 23. Cam 1 7 of the contact breaker rotates and opens thebreaker points causing the storage capacitor to discharge and energy tobe supplied to the plugs. System 10 also includes circuitry whereby thedischarge path from the energy storage capacitor is interrupted topermit capacitor recharging for the next succeeding spark interval.

A capacitive discharge ignition system 10 according to the presentinvention is shown in schematic form in FIG. 2. Power from a source suchas a l2-volt battery is supplied by an input connection 12 and a fourpin plug 14 on the unit. An inductor 16 is connected between the inputconnection 12 and an inverter or oscillator 29. Inverter 29 includes apair of transformers 20 and 24 having center tapped primary andsecondary windings, respectively, with the center taps being connectedto one another bya resistor 22. Inductor 16 is connected to the side ofresistor 22 adjacent transformer 20. A diode 26 connects the side ofresistor 22 opposite transformer 20 to ground. Inverter 29 furtherincludes a pair of transistors 31, 33 having their base electrodesconnected to opposite ends of the secondary winding of transformer 24,their collector electrodes to 36 to a second energy storage capacitor 38and through a parallel diode-78-resistor 80 combination to a thirdenergy storage capacitor 76. Each of capacitors 34, 38

and 76 have a capacitance value of typically between 1.5 and 2.5microfarads, and are in turn connected in common to one side of aninductor 35, the opposite of which is connected by a circuit connection37 through plug 14 to the primary winding 43 of ignition coil 42. Aninductor 74 is connected between the side of inductor 35 opposite thecommon connection thereof to the three energy storage capacitors 34, 38and 76 and a circuit ground or common point, The portion of the circuitjust described traces the path of energy supplied from the batterythrough the energy storage capacitor of the system of the presentinvention preparatory to discharge and the supplying of energy throughthe igni-.

tion coil to the spark plugs to provide combustion of the fuel mixturein the engine cylinders.

The control portion 39 of the circuitry of the present inventioncomprises a silicon controlled rectifier (SCR) 40 and a pulse shapingcircuit which is connected between a gate electrode 41 of SCR 40 andlead 44 extending through plug 1 4 to the breaker points 15. Controlcircuit 39 is also connected on one side by means of a circuitconnection from the anode 56 of the SCR to the side of resistor 32common to capacitor 34 at one side and on its other side to a resistor18 and the power input connection 12. When the breaker points areclosed, current flowing from the battery through plug 14 and resistor 18is shorted to ground through the points. When the points open, currentfrom the battery is directed through the pulse shaping circuitry 50 ofcontrol circuit 39 and thence to the gate electrode 41 of SCR 40 tocause the SCR to be turned on. A discharge path for energy in storagecapacitor 34 is thereby provided, the path including the primary 43 ofignition coil 42, inductor 35, capacitor 34 and SCR 40. Upon dischargeand energy flow, a high voltage pulse is induced in the secondarywinding 45 and an ignition spark produced at one of the engine sparkplugs.

The circuit also incorporates within itself a mode of operation forturning the SCR off after a surge of em ergy from capacitor 34 has beensupplied responsive to opening of the breaker points. The method ofturning the SCR off according to the present invention is to back-biasthe SCR, i.e., reverse the voltage so that the current attempts to go upthrough the SCR from ground toward capacitor 34. Since SCR 40, capacitor34 and the primary winding 43 of coil 42 which acts as an indicator arein series, there is a natural tendency for the desired back-biasing tooccur. After the initial spark energizing surge of power from storagecapacitor 34, the side of the capacitor adjacent coil 35 assumes apositive charge and has a tendency to drive current down through thecoil and up through the SCR (reverse direction), the net effect of thiscircuit action being to shut the SCR off.

To assure positive shut-ofi', it is necessary that the SCR beback-biased a predetermined minimum length of time (the specified SCRturnoff time typically is to 40 microseconds) after every discharge ofthecapacitor. To obtain such assurance, a coil of a predeterminedinductance is inserted in series between the primary 43 of coil 42 andthe energy storage capacitor 34. Due to the additional inductancecontributed by coil 35, the series SCR -capacitor 34-primary winding 43circuit now maintains its back-biasing polarity a sufficient length oftime to insure that the SCR will turn off even in the extreme case wherethe inductance of winding 43 goes to zero. Thus, despite any problemwhich may lower the inductance of the primary of coil 42, for example,water splashed on the coil, a short-circuit causing the hot side of thecoil to be grounded to the case, grease and rod grime build-up or carbontracking from the high voltage terminal, in short, any problem tendingto reduce the inductance of the primary of the ignition coil will notreduce the back-biased turnoff capability of the circuit.

Inductor 74 is connected to the junction of inductor 35 and primarywinding 43 for the purpose of further ensuring that the SCR is turnedoff.

Under certain circumstances (particularly at high en gine speeds), theprimary of the ignition coil acts as if itwere a veryhigh inductance andseverely retards the ability of capacitor 34 to discharge; Provision ofinductor 74 provides an inductance in parallel with primary winding 43and this limits the maximum inductance of the combination (even wherethe inductance of winding 43 appears to be infinite) to that of inductor74. By proper choice of the inductance value of coil 74 (1.0 to 3.0millihenries when used with commercially available ignition coils),discharge of the energy storage capacitor is still accomplished therebyassuring sufiicient back-biasing current for SCR turnoff during theimmediately subsequent back-biasing portion of system operation.

' Inductor 35 provides an additional and important contribution to theignition system by acting as a device for limiting current to the SCR.By providing an inductor of a predetermined magnitude, e.g., 270microhenries, the current through the SCR is maintained at a maximum of60 amps or less, even where the inductance of winding 43 has gone tozero. Again, even under the most adverse circumstances of circuitoperation, the current to the SCR is limited to a value which can easilybe absorbed by the SCR. The net result of the provision of inductors 74and 35 is that no change in ignition coil properties can cause the SCRto fail to turn off or tocause it to be damaged due to current surges.

The circuit of the present invention also incorporates the capability ofutilizing the transient energy remaining in thecircuit subsequent toeach storage capacitor discharge to recharge capacitor 34 to a partialvalueof its total charge without the necessity of drawing energy fromthe oscillator 28 (inverter). Harnessing the residual energy in acircuit to partially recharge the energy storage capacitor is beneficialin reducing wear on the inverter circuit and battery current drain bydrawing less current therefrom as well as in checking spark plug erosionwere the transient energy not channeled back to capacitor 34.

The foregoing is accomplished by providing a pair of silicon diodes 52and 54 (approximately 1 amp., 800

PW) in series connected between a ground connection and the anode 56 ofSCR 40. Upon turning on the-SCR, a discharge path to ground is providedand current flows up through the primary of coil 42 and inductor 35until the potential on the side of capacitor 34 adjacent coil 35 isreduced to zero. At this instant a significant amount of energy isstored in the primary of coil 42 and in inductor 35 which, unlessdissipated in some constructive manner, produces undesirable oscillationin the ignition system. current continues to flow to winding 43 andinductor 35 and capacitor 34 charges in the opposite direction. At itsmaximum opposite charge (approximately +350 volts relative to inductor35) the current in inductor 35 and winding 42 iszero. The reverse chargeon capacitor 34 then begins to drive current in the opposite directionthrough coil 35 and winding 42. By providing diodes 52 and 54, a currentpath is provided such that when the current reverses and begins to flowin the opposite direction through the inductor 35 and the primary ofcoil 42, storage capacitor 34 is thereby recharged to approximatelypercent of .its fully charged value without drawing power from theinverter. The current path through diodes 52, 54 also provides a meanswhereby any tendency of the coil to build up residual magnetism isreduced or eliminated as well as tending to protect against ignitioncoil insulation breakdown. (At the end of each discharge cycle thecurrent through the coil has traced one nearly perfect sine wavepattern.)

Diodes 52 and 54perform a third function in assuring that the SCR 40 isshut off. Because there is approximately a 1 volt drop across each ofthe diodes and the cathode 58 of SCR 40 is permanently connected toground, current up through diode 54 from ground (during the reversecurrent portion of circuit operation) produces a minus one volt (-l.0v.)potential with respect to ground on the gate 41 of the SCR and diode 52produces a second 1 volt drop, simultaneously placing the anode at apotential of minus (-2.0v. with respect to ground and at a potential ofminus (l-.0v.) withrespect to the gate electrode 41. The SCR is thusfully back-biased and is thereby shut off in the minimum' time possible.

A further advantage of the circuit of the presentinvention is itsability to channel and-dissipate high voltage energy spikes which wouldotherwise have the tendency to damage or destroy the SCR. Under normaloperating conditions, when energy is supplied to the spark plug and, anarc-over occurs, the energy supplied by the storagecapacitor isdissipated in the spark plug gap and only a relatively small amount ofenergy is left in the secondary 45 of the ignition coil. However, whenthe circuit fails to produce a spark at the plug (no arcover), e.g.,when spark plug condition has seriouslydeteriorated, the energy whichwould ordinarily have been transmitted to the spark plug gap is storedas an extremely high voltage by the capacitive action of the spark plugwires and the ignition coil secondary. If not dissipated, this energy isreflected back to the ignition coil primary 43, capacitor 34 and SCR 40as a high voltage spike. If a sufiicient number of such high voltagespikes were'allowed to be transmitted to the SCR, serious deteriorationof the SCR results to the point where the SCR ultimately stopsfunctioning.

Such an occurrence is prevented in the present circuit by inductor 35which reflects approximately percent of any pulse transmitted fromwinding 43 back of the pulse exhausts itself by bouncing back and forthbetween inductor 35 and coil 43. This is the third function of inductor35.

The remaining 20 percent of the energy is transmitted through inductor35 and capacitor 34 and toward SCR 40. This transmitted energy is of amagnitude that conventional means of suppressing high voltage spikes(e.g., zener diodes, transient suppressors, small (0.01 microfarad)capacitors in parallel with the device to be protected) are inadequate.The problem is further complicated by the fact that the transientvoltage is the same polarity as the original charging voltage, but beingapproximately an order of magnitude higher in value, it would eventuallydestroy the SCR even if the SCR were selected so as to have a voltagerating that was a multiple of the original capacitor charging voltage.

Use of diode 60 in conjunction with a large capacitor 62 (on the orderof 2 microfarads) provides the advantages of a large capacitor inparallel with the SCR for high energy voltage spike protection while atthe same time eliminating the disadvantages of extremely high SCRcurrents and high energy requirements from the.

power supply inherent in the use of a large, direct cou pled parallelcapacitor. When the high voltage, spike is transmitted through capacitor34 and inductor 35 it forward-biases diode 60 which couples the largefilter capacitor 62 in parallel with SCR and thereby harmlessly absorbsthe transient voltage spike. When the SCR turns on, diode 60 becomesback-biased thereby isolatingcapacitor 62 from the SCR and preventing itfrom discharging into the SCR.

Accidental firing of SCR 40 which could cause a premature enginedamaging spark is prevented by the provision of several circuitcomponents in a specific arrangement. In the first instance, a filtercomprised of inductor 16 (5O microhenries) and capacitor 64 (1Omicrofarads or greater) filters out any voltage spikes generated byinverter 20 and prevents such spikes from being transmitted to the SCR.The same filtering action exerted by inductor 16 and capacitor 64 alsoprevents any noise in the form of voltage spikes from the power sourcefrom being transmitted to the inverter, an important precaution inpreventing such voltage spikes from passing through the inverter to theanode of the SCR.

A second precaution against premature SCR firing resides in the designof the pulse shaping circuit 50. As indicated earlier, the signal istransmitted to the gate electrode 41 of SCR 40-at the instant the pointsor breaker contacts open. When the points open, current flows throughresistor 18 and through the parallel combination of resistor 46 anddiode 48 and begins to charge capacitors 66 (l microfarad) and 68 (0.1microfarad). The charging time constant (1.] milliseconds) of capacitors66 and 68 is chosen such that it al- 8 acts to shortcircuit such spikesarriving at that point in the circuit to ground.

In addition to blocking all voltage spikes above a predeterminedvoltage, the parallel combination of resis tor 46 and diode 48 alsoprevents point bounce from firing the SCR. Since point bounce occursimmediately after the points close and a signal has been transmittedthrough circuit 50 to the SCR, capacitors 66 and 68 are still in acharged condition and, due to the magnitude of resistor 46 (providing atime constant of l. l milliseconds), have not had a chance to discharge.Therefore, voltage spikes generated due to point bounce will not betransmitted to the SCR since it is the process of charging capacitor 68which causes the SCR to fire. The condition of capacitor 68 alreadybeing charged thus prevents transmission of the spike to the SCR. otherrandom voltage spikes introduced into the circuit, e.g., from the 12volt source, are also not transmitted to the SCR because such spikes arediverted to pass through resistor 18 and through the closed set ofbreaker points to ground. By choosing resistor 18 of a sufficiently lowvalue (20 ohms), electromagnetic pickup is not a problem and theresistor further determines the amount of current through the closedpoints such that sufficient heat is generated to keep the points cleanbut is limited to a value which will not produce significant wear.

An important characteristic of the ignition system of the presentinvention is its ability to produce a variable power pulse to thesparkplugs of the engine to suit varying engine conditions. Among otherconditions enlows enough current to pass through capacitor 68 to firethe SCR even at very low temperatures (35F.) but nevertheless will notpass voltage spikes over capacitor 66 and through capacitor 68 to thegate of the SCR. 4

Circuit 50 is also provided with a second stage comprising resistor 72and capacitor 70. Assuming for the moment the possibility that a voltagespike does pass capacitor 66 and resistor 72, capacitor 70, which isconnected in parallel circuit relationship with diode 54,

countered in a normal operation are a cold engine upon starting, atendency of the spark plugs to foul when running the engine at low oridling speeds and wear and deterioration of the various components ofthe ignition system including the ignition coil and spark plugs. Withrespect to the first condition, it takes considerably more spark energyto start a cold engine than to run it, once warm, and, therefore, asubstantially higher voltage from the energy storage capacitor duringthis interval is desirable. The starting problem is further compoundedby the fact that the battery voltage is normally relatively low atthespecifictime when it is required that the energy storage capacitorvoltage be high. As concerns the second condition it is also desirableto provide more energy'to the spark plugs at low speeds since suchincreasedenergy has a tendency to burn fouling material which may begenerated. In addition, if such fouling material does become deposited,there is still sufficient spark energy to fire the fuel mixture in thecylinder despite the energy drain caused by the presence of thismaterial. As the following discussion will disclose, specificportions ofthe ignition system of the present inventionhave been designed tofulfill these desired requirements.

By providing a saturating type of core material for transformers 20 and24 of inverter 29, both of these transformers are provided with thecharacteristic that they are voltage and current dependent. In oneexample this is accomplished by providing transformer 20 with a steelcore of 14 gauge E-l laminations and transformer 24 with a toroid coreof semi-square hysteresis loop material as used in commercial ferritessuch as Ferroxcube 3E material. This feature is utilized to provide aninverter having a voltage characteristic which varies according to thevarying requirements of the engine with which it is being used to varythe electric power supplied to the storage capacitor. Thus, a highervoltage on starting (engine cranking speeds) is provided, with asomewhat reduced voltage when the engine is idling or running slowly(low and intermediate engine speeds) and a still further reduced voltageis provided when engine revolutions have risen to a higher speed (normalengine running speeds) at which gasoline wetting of plugs is normallynot a problem. In a typical case, the output voltage from the inverterfor engine RPMs from O to 250 (starting) with an input voltage from thebattery of from 7.5 to 16 volts is approximately 600 volts. When theengine RPMs are from 400 to 750, that is, idling and very low speed, theoutput voltage of the inverter is inversely proportional to engine speedin the range of 600 volts to approximately 425 volts. When the engineRPMs increase to a value above 750, the inverter output voltage furtherdrops to approximately 425 volts and remains at that value over theentire range of engine running speeds, thereby serving to maintain sparkenergy constant over this entire range. In contrast, in conventionalKettering ignition systems, spark energy decreases as engine RPMsincrease, a serious disadvantage of such systems.

As a further means of increasing spark energy, for example, in thesituation of starting a cold engine, the circuit of the presentinvention increases spark energy and spark duration by increasing thevalue of the energy storage capacitor when the engine is cold. This isaccomplished by providing a temperature sensitive switch 36 which isphysically located in position adjacent to resistor 32. Since current inthis resistor is proportional to engine RPMs, resistor 32 warms up atthe rate which closely approximates engine speed. Since the engine warmsup at a rate roughly proportional to the square of its speed, theincrease in'temperature of resistor 32 closely approximates the increasein engine temperature. When the temperature is below a critical value(e.g., 100F.) and increasing, switch 36 is closed and capacitor 38 isconnected into the circuit increas-' ing the available capacitance ofthe energy storage capacitor. After resistor 32 has warmed, and likewisethe engine has warmed to the proper temperature (e.g., 135F.), switch 36opens and the value of the discharge capacitance is reduced by removingcapacitor 38 from the circuit. During engine cooling, switch 36 closesat an engine temperature of a predetermined value (e.g., 96F Increasingthe value of capacitance not only increases the amount of energyavailable for delivery to the spark plugs but also lengthens theduration of the ignition spark when the engine is cold due to theincreased oscillatory period of a circuit having an enlarged capacity.

To provide an additional increase in spark energy and duration at lowengine speeds (above the increased energy supplied by increased inverteroutput voltage at these speeds), a series-parallel circuit comprising acapacitor 76 in series with a parallel combination of a diode 78 and aresistor 80 is connected between the junction of resistor 32 andcapacitor 34 on one side and between thejunction of capacitors 34 and 38on the opposite side thereby connecting capacitor 76 in parallel circuitrelationship with capacitors 34, 38. The purpose of this circuitcombination is to produce an additional measure of capacitance by meansof capacitor 76 at low engine speeds (regardless of engine temperature)with the contribution of this portion of the circuit diminishing toessentially zero as engine RPMs reach and exceed 1,000. The operation ofthis portionof the circuit is as follows: At low engine speeds, as theSCR fires, capacitors 34 and 76 (and capacitor 38- when it is connectedin the circuit) discharge providing an increase in energy to the plugs,the discharge of capacitor 76 being obtained through diode 78. Byselective choice of the value of resistor 80, a time constant of this RC80, 76 combination (e.g., 20 milliseconds) can be obtained such that atlow engine speeds a significant charge can be built upon capacitor 76but at higher ongine speeds, discharge of capacitor 34 occurs sofrequently as to prevent significant charge from being accumulated oncapacitor 76.

Inductor 30 of the circuit of the present invention enhances the abilityof the circuit to generate a spark despite the fact that the spark plugsmay be fouled or wetted. The circuit action is accomplished because theSCR stays on for a longer period of time due to the fact that theeffective inductance and resistance of the coil is increased beforearc-over of the plug occurs and areover in a foul or wetted plug alwaystakes longer than in a clean plug. By virtue of the SCR staying onlonger, a voltage exists across inductor 30 for a longer period of timeand thus a significant amount of energy is stored therein which, whenthe SCR is shut off, is transmitted through the plug gap, tending toflash or burn up the contaminating materials; the remaining portion ofthe energy in inductor 30 is transmitted to capacitor 34 to charge it ata rapid rate making it ready to provide a full measure of energy uponits next discharge despite being heavily drained on the previousdischarge by the contaminated plug.

There is thus provided an ignition system in which the SCR is protectedagainst damage from any eventuality, e.g., voltage or current surges,and further, a system is provided which adapts to changing engineconditions providing more or less energy as needed assuring maximumignition capabilities and minimum wear on all system parts as well asminimum plug erosion.

-Whereas an ignition system designed to deliver maximum energy to thespark plugs at all times would perform satisfactorily in terms of itsignition capabilities, the same wouldnot be true of its effect on thelife and erosion rate of the spark plugs. By incorporating the featuresoutlined in the preceding to automatically adjust and control the amountof energy delivered to the plugs during the various conditions 'to beencountered by the engine, the system of the present invention providesoptimum amounts of energy for each such condition. The life of theplugs'is thereby extended man interval which is several times greaterthan the spark plug lifetime in conventional ignition systems or in anysystem which does not jva'ry spark duration and intensity to suit engineneedsj What is claimed is: i 1. An ignition system for internalcombustion engines comprising:

a source of electric power; a first capacitor for storing electric powercoupled to the power source; a second capacitor connected in parallelcircuit relationship with said first capacitor; normally closedswitching means connected in series circuit relationship with the secondcapacitor; temperature sensing means operatively associated with saidswitching means for opening said switchl1 l2 ing means when enginetemperature is above precircuit means coupled to said third capacitordetermined threshhold value to disconnect said whereby the charging rateof said third capacitor is second capacitor from the ignition system;controlled such that additional capacitance is only an ignition coilcoupled to said first and second caprovided in the system at crankingand low engine pacitors; 5 speeds. means coupled to the ignition coilfor producing an 3. An ignition system according to claim 2 whereinelectric spark; the circuit means includes a resistor connected in paranelectronic switch coupled to the first and second allel circuitrelationship with said first and second cacapacitor for controlling thedischarge and charge pacitors, the resistor and third capacitor having atime of the first and second capacitors; and constant for providing saidadditional capacitance at engine controlled switching means coupled tothe said engine speeds.

electronic switch for periodically operating said 4. An ignition systemaccording to claim 1 including switch to produce discharge of thecapacitors. means for closing said switching means when engine 2. Anignition system according to claim 1 including temperature drops below asecond predetermined a third capacitor connected in parallel circuitrelationthreshold value lower than said first value. ship with saidfirst and second capacitors; and

1. An ignition system for internal combustion engines comprising: asource of electric power; a first capacitor for storing electric powercoupled to the power source; a second capacitor connected in parallelcircuit relationship with said first capacitor; normally closedswitching means Connected in series circuit relationship with the secondcapacitor; temperature sensing means operatively associated with saidswitching means for opening said switching means when engine temperatureis above predetermined threshhold value to disconnect said secondcapacitor from the ignition system; an ignition coil coupled to saidfirst and second capacitors; means coupled to the ignition coil forproducing an electric spark; an electronic switch coupled to the firstand second capacitor for controlling the discharge and charge of thefirst and second capacitors; and engine controlled switching meanscoupled to the electronic switch for periodically operating said switchto produce discharge of the capacitors.
 2. An ignition system accordingto claim 1 including a third capacitor connected in parallel circuitrelationship with said first and second capacitors; and circuit meanscoupled to said third capacitor whereby the charging rate of said thirdcapacitor is controlled such that additional capacitance is onlyprovided in the system at cranking and low engine speeds.
 3. An ignitionsystem according to claim 2 wherein the circuit means includes aresistor connected in parallel circuit relationship with said first andsecond capacitors, the resistor and third capacitor having a timeconstant for providing said additional capacitance at said enginespeeds.
 4. An ignition system according to claim 1 including means forclosing said switching means when engine temperature drops below asecond predetermined threshold value lower than said first value.